[RE-wrenches] Modified sine deterioration

[Exeltech]

Hello Drake,
 
As a professional electrial design engineer in the solar energy industry, I can say without hesitation, the simple answer to your question is "Yes".  Waveforms can deterioriate as components age.  Lower-quality capacitors will dry out in 20 years' use (or less!), and this in turn can indeed alter the waveform.  You'd need at a minimum a good oscilloscope to observe the waveform.  If it's really ragged, and no longer stair-stepped, the inverter is clearly suspect.  If it resembles a "Pyramid", there may be another issue afoot.
 
Motors connected to modified square wave power turn at lower RPM and operate [sometimes much] hotter than their sine-fed bretheren.  Over time, the increased heat takes its toll on the wire insulation in the motor, causing an eventual breakdown of the insulation and a short-circuit of the windings.  Presto!  Motor BBQ (sometimes inverter too).
 
 
By the way .. to use "quality" in context with reference to a modified square wave is quite an oxymoron .. but in this case, it fits.
 
 
Dan
 
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On Tue, 5/20/14, Drake <drake.chamberlin at redwoodalliance.org> wrote:

Subject: [RE-wrenches] Modified sine deterioration
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Date: Tuesday, May 20, 2014, 9:24 AM

Hello Wrenches,

We are diagnosing a system with a 20 year old Trace DR 1512 inverter. Recently a motor burned out that had been running on the system. I do understand that modified sine (modified square) wave inverters are hard on motors in general.

My question is, "does the quality of the Mayan Pyramid sine wave deteriorate with time as the capacitors dry up, or for any other reason?  Would the power quality be expected to have declined on an old unit like this?

Thanks,

Drake

Drake Chamberlin
Athens Electric LLC
OH License 44810
CO License 3773
NABCEP Certified Solar PV
740-448-7328
http://athens-electric.com/